Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!
back to [[Statistical Dispersion]] The '''variance''' of a set of data is the mean of the sum of squared deviations from the [[Arithmetic Mean]] of the same set of data. Because this calculation sums the squared deviations, the unit of variance is the square of the unit of observation. Thus, the variance of a set of heights measured in inches will be given in square inches. This fact is inconvenient and has motivated statisticians to call the square root of the variance, the [[Standard Deviation]] and to quote this value as a summary of dispersion. When the set of data is a [[Population]], we call this the ''population variance''. If the set is a [[Sample]], we call it the ''sample variance''. The method of calculation is easily understood from the table below where the mean is 8.i x[i] x[i]-mean (x[i]-mean)^2 1 5 -3 9 2 7 -1 1 3 8 0 0 4 10 2 4 5 10 2 4 --- ---- --- --- n=5 sum=40 0 18 mean = 40/5 = 8 variance = 18/5 = 3.6 standard deviation = 1.897366596101Note that the column of deviations sums to zero. This is always the case. ---- [[Dick Beldin]]